1. Field of the Invention
This invention relates to a pack assembly for use in hot rolling a material sensitive to heat loss, such as gamma titanium aluminide.
2. Description of the Prior Art
For various applications where high strength-to-weight ratios are required at elevated temperatures, such as jet engine and air frame applications, it is known to employ gamma titanium aluminides. These materials include a titanium-aluminum intermetallic compound of gamma titainium aluminide and may comprise titanium in combination with 31-35 weight percent aluminum with optional additions of beta stabilizers, such as vanadium, chromium, molybdenum, tungsten, tantalum, niobium, and manganese within the range of 0.25 to 12%.
Although gamma titanium aluminide alloys find useful application in the form of flat-rolled product, such as sheet, plate and foil, they are deformation temperature sensitive in that they tend to crack upon deformation, such as by rolling, if proper elevated temperatures are not maintained. The common practice used to produce these flat rolled products from conventional titanium alloys is to forge an ingot of the alloy to a slab and hot roll the slab to a thin gage, for example 0.020 to 0.25 inch thicknesses. When thicknesses are below approximately 0.2 inch a common practice is to use a `pack rolling ` method whereby steel cover plates and side bars are welded together around the titanium workpiece(s). This practice helps to minimize the radiant heat loss and conductive heat loss caused by the use of cold (less than 300.degree. F.) rolls which are considerably lower in temperature than the workpiece. The hot working temperature employed during these operations is usually dictated by the beta transus temperature of the particular alloy composition. Most conventional titanium-base alloys have beta transus temperatures within the range of 1700.degree. to 1900.degree. F. and consequently typical working temperatures for these alloys are within the range of 1500.degree. to 2100.degree. F. Gamma titanium aluminide alloys, however, have beta transus temperatures within the range of 2400.degree. to 2550.degree. F., and thus require much higher than typical hot working temperatures. In addition, it is known that maintenance of these high temperature during rolling is important.
The problems of working gamma titanium aluminides by conventional hot working practices therefore include the practical temperature limitations of the furnaces employed for heating to hot rolling temperature, the cooling effect imparted to the alloy during hot rolling by the work rolls of the rolling mill and natural radiant heat loss.
Consequently, with conventional hot-rolling techniques gamma titanium aluminides are subject to cracking during the hot rolling operation, particularly when hot rolled to relatively thin gages where greater heat losses occur. Also due to the high working temperatures required for rolling gamma titanium aluminides, conventional pack materials (such as steels) are not useful due to the eutectic reaction which occurs when titanium is in contact with iron or nickel based materials above about 2000.degree. F. Thus, a non-reactive pack material is also required, in addition to a pack construction which minimizes heat loss.